 Index
 ImageMagick Examples Preface and Index
 Introduction
 The Fourier Transform
 FFT/IFT In ImageMagick
 Properties Of The Fourier Transform
 Practical Applications
 Advanced Applications

 FFT Multiplication and Division (low level examples  subpage)
Introduction
One of the hardest concepts to comprehend in image processing is Fourier
Transforms. There are two reasons for this. First, it is mathematically
advanced and second, the resulting images, which do not resemble the original
image, are hard to interpret.
Nevertheless, utilizing Fourier Transforms can provide new ways to do familiar
processing such as enhancing brightness and contrast, blurring, sharpening and
noise removal. But it can also provide new capabilities that one cannot do in
the normal image domain. These include deconvolution (also known as
deblurring) of typical camera distortions such as motion blur and lens defocus
and image matching using normalized cross correlation.
It is the goal of this page to try to explain the background and simplified
mathematics of the Fourier Transform and to give examples of the processing
that one can do by using the Fourier Transform.
If you find this too much, you can skip it and simply focus on the properties
and examples, starting with
FFT/IFT In ImageMagick
For those interested, another nice simple discussion, including analogies to
optics, can be found at
An Intuitive
Explanation of Fourier Theory.
The lecture notes from Vanderbilt University School Of Engineering are also
very informative for the more mathematically inclined:
1 & 2 Dimensional Fourier Transforms and
Frequency Filtering.
Other mathematical references include Wikipedia pages on
Fourier Transform,
Discrete
Fourier Transform and
Fast Fourier
Transform as well as
Complex Numbers.
My thanks to Sean Burke for his coding of the original demo and to
ImageMagick's creator for integrating it into ImageMagick. Both were heroic
efforts.
Many of the examples use a
HDRI Version of
ImageMagick which is needed to preserve accuracy of the transformed
images. It is recommened that you compile a personal HDRI version if you want
to make the most of these techniques.
The Fourier Transform
An image normally consists of an array of 'pixels' each of which are defined
by a set of values: red, green, blue and sometimes transparency as well. But
for our purposes here we will ignore transparency. Thus each of the red, green
and blue 'channels' contain a set of 'intensity' or 'grayscale' values.
This is known as a raster image '
in the spatial domain'. This is just
a fancy way of saying, the image is defined by the 'intensity values' it has
at each 'location' or 'position in space'.
But an image can also be represented in another way, known as the image's
'
frequency domain'. In this domain, each image channel is represented
in terms of sinusoidal waves.
In such a '
frequency domain', each channel has 'amplitude' values that
are stored in locations based not on X,Y 'spatial' coordinates, but on X,Y
'frequencies'. Since this is a digital representation, the frequencies are
multiples of a 'smallest' or unit frequency and the pixel coordinates
represent the indices or integer multiples of this unit frequency.
This follows from the principal that "any wellbehaved function can be
represented by a superposition (combination or sum) of sinusoidal waves". In
other words, the 'frequency domain' representation is just another way to
store and reproduce the 'spatial domain' image.
But how can an image be represented as a 'wave'?
Images are Waves
Well if we take a single row or column of pixel from
any image, and
graph it (generated using "gnuplot" using the script "
im_profile
"), you will find that it
looks rather like a wave.
convert holocaust_tn.gif colorspace gray miff: \
im_profile s  image_profile.gif

If the fluctuations were more regular in spacing and amplitude, you would get
something more like a wave pattern, such as...
convert size 20x150 gradient: rotate 90 \
function sinusoid 3.5,0,.4 wave.gif
im_profile s wave.gif wave_profile.gif

However while this regular wave pattern is vaguely similar to the image profile
shown above, it is too regular.
However if you were to add more waves together you can make a pattern that is
even closer to the one from the image.
convert size 1x150 gradient: rotate 90 \
function sinusoid 3.5,0,.25,.25 wave_1.png
convert size 1x150 gradient: rotate 90 \
function sinusoid 1.5,90,.13,.15 wave_2.png
convert size 1x150 gradient: rotate 90 \
function sinusoid 0.6,90,.07,.1 wave_3.png
convert wave_1.png wave_2.png wave_3.png \
evaluatesequence add added_waves.png

See also
Adding Biased Gradients
for a alternative example to the above.
This '
wave superposition' (addition of waves) is much closer, but still
does not exactly match the image pattern. However, you can continue in this
manner, adding more waves and adjusting them, so the resulting composite wave
gets closer and closer to the actual profile of the original image. Eventually
by adding enough waves you can exactly reproduce the original profile of the
image.
This was the discovery made by the mathematician
Joseph Fourier. A
modern interpretation of which states that "any wellbehaved function can be
represented by a superposition of sinusoidal waves".
In other words by adding together a sufficient number of sine waves of just
the right frequency and amplitude, you can reproduce any fluctuating pattern.
Therefore, the 'frequency domain' representation is just another way to
store and reproduce the 'spatial domain' image.
The '
Fourier Transform' is then the process of working out what 'waves'
comprise an image, just as was done in the above example.
2 Dimensional Waves in Images
The above shows one example of how you can approximate the profile of a single
row of an image with multiple sine waves. However images are 2 dimensional,
and as such the waves used to represent an image in the 'frequency domain'
also needs to be two dimensional.
Here is an example of one such 2 dimensional wave.
The wave has a number of components to it.
Image example
Using FFT/IFT In ImageMagick
Implementation Notes
ImageMagick makes use of the
FFTW, Discrete
Fourier Transform Library which requires images to be converted to and
from floating point values (complex numbers), and was first implemented in
IM version 6.5.43.
To make it work as people generally expect for images, any nonsquare image or
one with an odd dimension will be padded (using
Virtual Pixels to be square of the maximum width or height of the image.
To allow for proper centering of the 'FFT origin' in the center of the image,
it is also forced to have a even (multiple of 2) dimensions. The consequence
of this is that after applying the Inverse Fourier Transform, the image will
need to be cropped back to its original dimensions to remove the padding.
As the Fourier Transform is composed of "
Complex Numbers", the
result of the transform cannot be visualized directly. Therefore, the complex
transform is separated into two component images in one of two forms.
Complex Number
Real/Imaginary

Real and Imaginary
The normal mathematical and numerical representation of the "
Complex Numbers", is
a pair of floating point values consisting of 'Real' (a) and 'Imaginary' (b)
components. Unfortunately these two numbers may contain negative values and
thus do not form viewable images.
As such this representation can not be using in a normal version of
ImageMagick, which would clip such images (see example below of the resulting
effects.
However when using a
HDRI version of
ImageMagick you can still generate, use, and even save this representation
of a Fourier Transformed image. They may not be useful or even viewable as
images in their own right, but you can still apply many mathematical
operations to them.
To generate this representation we use the 'plus' form of the operators,
"
+fft
" and "
+ift
", and will be looked at in
detail below in
FFT as RealImaginary Components.
Complex Polar
Magnitude/Phase

Magnitude and Phase
Direct numerical representation of the "
Complex Numbers" is
not very useful for image work. But by plotting the values onto
a 2dimensional plane, you can then convert the value into a
Polar
Representation consisting of 'Magnitude' (r) and 'Phase' (θ)
components.
This form is very useful in image processing, especially the magnitude
component, which essentially specifies all the frequencies that go to make up
the image.
The 'Magnitude' component only contains positive values, and is just directly
mapped into image values. It has no fixed range of values, though except for
the DC or zero frequency color, the values will generally be quite small. As
a consequence of this the magnitude image generally will appear to be very
dark (practically black).
By scaling the magnitude and applying a log transform of its intensity values
usually will be needed to bring out any visual detail. The resulting
'logtransformed' magnitude image is known as the image's 'spectrum'. However
remember that it is the 'magnitude' image, and not the 'spectrum' image, that
should be used for the inverse transform.
The DC (Short for "Direct Current") or "Zero Frequency", color that appears at
the central 'origin' of the image, will be the average color value for the
whole image. Also as input images not contain 'imaginary' components, the DC
phase value will also always have zero phase, producing a puregray color.
The 'Phase' component however ranges from π to +π. This is first biased
into a 0 to 2π range, then scaled into actual image values ranging from
0 to
QuantumRange (as determined by the
Compile Time Memory Quality). As a consequence of this, a zero phase
will have a puregray value (as appropriate for each channel), while a negated
phase will be a pureblack ('
0
') value. Note that a purewhite
('
QuantumRange
') is almost but not quite the same thing.
A Magnitude and Phase FFT representation of an image is generated using the
normal FFT operators, "
+fft
" and "
+ift
". This will be looked at first in
Generating
FFT Images and its Inverse.
Generating FFT Images and its Inverse
(Magnitude and Phase)
Now, lets simply try a Fourier Transform round trip on the Lena image. That
is, we simply do the forward transform and immediately apply the inverse
transform to get back the original image. Then we will compare the results to
see the level of quality produced.
time convert lena.png fft ift lena_roundtrip.png
echo n "RMSE = "
compare metric RMSE lena.png lena_roundtrip.png null:
echo n "PAE = "
compare metric PAE lena.png lena_roundtrip.png null:

The "
compare
" program above
returns a measure of how different the two images are. In this case you can
see that the general difference is very small, of about
0.22%
.
With a peak value difference in at least one pixel of about
("
PAE
", Peak Absolute Error) of just about
1%
.
You can improve this by using a
HDRI version of
ImageMagick. (See
FFT with HDRI below).
Lets take a closer look at the FFT images that were generated in the above
round trip.
convert lena.png fft +depth +adjoin lena_fft_%d.png

As
John M.
Brayer said about Fourier Transforms...
We generally do not display
PHASE images because most people who see them shortly thereafter succumb to
hallucinogenics or end up in a Tibetan monastery.
Note that the "
fft
"
operator generated two images, the first image is the 'magnitude' component
(yes it is mostly black with a single colored dot in the middle), while the
second, almost random looking image, contains the 'phase' component.
PNG images can only store one image per file as such neither the "
+adjoin
" or the '
%d
'
in the output filename was actually needed, as IM would handle this.
However I include the options in the above for completeness, so as to make it
clear I am generating two separate image files, not one. See
Writing a MultiImage Sequence for more details.
As two images are generated the magnitude image (first of zeroth image) is
saved into "
lena_fft_0.png
" and phase image (second image) into
"
lena_fft_1.png
".

To prevent any chance of distortions resulting from saving FFT images, It is
best not to save them to disk at all, but hold them in memory while you
process the image.
If you must save then it is best to use the Magick File Format "MIFF " so as to preserve the image at its
highest quality (bit depth). This format can also save multiple images in
the one file. For script work you can also use the verbose "TXT " Enumerated Pixel Format.
DO NOT save them using "JPEG ", "GIF " image
formats.
If you must save these images into files for actual viewing, such as for
a web browser, use the image format "PNG " with a "+depth " reset to the internal default, (as we do in these
examples). However it can only store one image per file.
The "TIFF " file format can also
be used though is not as acceptable for web browsers, though it does allow
multiple images per file.

The best way of saving intermediate images into a single file is to use the
"
MIFF
", file format...
convert lena.png fft +depth lena_fft.miff

Or you can save them into complete separate filenames using "
write
" (See
Writing Images)...
convert lena.png fft +depth \
\( clone 0 write lena_magnitude.png +delete \) \
\( clone 1 write lena_phase.png +delete \) \
null:

Note that in the above I used the special "
NULL:
" image format to junk the two images which are still
preserved in memory for further processing.
And finally we again read in the two images again, so as to convert it back
into a normal 'spatial' image...
convert lena_magnitude.png lena_phase.png ift lena_restored.png

Both images generated by the FFT process are very sensitive to modification,
where even small changes can result greatly distorted results. As such it is
important to never save them in any image format that could distort those
values.
It important to remember that both images are needed when recovering the image
from the frequency domain. So it is no good saving one image, and junking the
other, if you plan on using them for image reconstruction.
Magnitude or Phase Only Images
Finally, lets try reconstructing an image from just its magnitude component or
just its phase component.
convert lena_fft_0.png size 128x128 xc:'gray(50%)' \
ift lena_magitude_only.png
convert size 128x128 xc:gray1 lena_fft_1.png ift lena_phase_only.png

Magnitude Only

Phase Only

You will note from this that it is the phase image that actually contains most
of the position information of the image, while the magnitude actually holds
much of the color information. This is not exact, as there is some overlap in
the information, but that is generally the case.
The 'Magnitude Only' image will always have white corners, as a constant 50%
phase image was used. You can remove those white patches by using a randomized
phase image. However make sure the phase of the center pixel is a perfect 50%
gray, or the whole image will be dimmed.
The 'Phase Only' image used a constant 1% gray (almost pure black) magnitude
image for the conversion. Even with this constant magnitude it still produces
patches of very intense pixels, especially along edges.
You just need to remember that both images are needed to reconstruct the
original image.
Frequency Spectrum Image
You will have noted that the magnitude image (the first or zeroth image),
appears to be almost totally black. It isn't really, but to our eyes all the
values are very very small. Such an image isn't really very interesting to
look at to study, so lets enhance the result with a log transform to produce
a '
frequency spectrum' image.
This is done by applying a strong
Evaluate Log Transform to a
Normalized 'magnitude' image.
convert lena_fft_0.png autolevel evaluate log 10000 \
lena_spectrum.png

Now we can see the detail in the spectrum version of the magnitude image.
You may even see some specific colors in the spectrum image, but generally
such colors are unimportant in a spectrum image. It is the overall intensity
of each frequency, and the patterns they produce that is far more important.
As such you may also like to grayscale the spectrum image after enhancement.
How much of a log enhancement you need to use depends on the image, so you
should adjust it until you get the amount of detail you need to clearly see
the pattern of the images frequency spectrum.
Alternatively you can use the following small shell script, to calculate
a log scaling factor to use for the specific magnitude image.
scale=`convert lena_fft_0.png autolevel \
format "%[fx:exp(log(mean)/log(0.5))]" info:`
convert lena_fft_0.png autolevel \
evaluate log $scale lena_spectrum_auto.png
 

However remember that you can not use a spectrum image, for the inverse
"ift " transform as it will
produce an overly bright image.
convert lena_spectrum.png lena_fft_1.png ift lena_roundtrip_fail.png

Basically as you have enhanced the 'magnitude' image, you have also enhanced
the resulting image in the same way, producing the badly 'clipped' result
shown.


HDRI FFT Images
When we mapped the results of the Fourier Transform into a image
representation we scaled and converted the values from floating point "
Complex Numbers", into
integer image values. This naturally produced
Rounding Errors, and other "Quantum"
Effects, especially in the smaller lower frequency magnitudes.
If accuracy is important in your image processing, then you will either need
to use a
Bit Quality (such as Q32 or Q64 bit
versions of ImageMagick), or better still use a
HDRI version ImageMagick so that the values are stored as floating point
numbers.
When using a HDRI version of IM with a
Magnitude and
Phase representation of the Fourier transform, the magnitude component
will still all be positive values, and as such can still be used as shown
above, just much more exactly. The phase component will however still be
biased and scaled, as previously shown.
In other words, the magnitude and phase representation in HDRI is exactly the
same, just much more accurate.
For example here I use a
HDRI version
ImageMagick to generate another 'round trip' conversion of an image.
# HDRI version of IM used
time convert lena.png fft ift lena_roundtrip_hdri.png
echo n "RMSE = "
compare metric RMSE lena.png lena_roundtrip_hdri.png null:
echo n "PAE = "
compare metric PAE lena.png lena_roundtrip_hdri.png null:
 

If you compare the results above with the previous nonHDRI comparison...
You will see that the HDRI version of IM produced a much more accurate result,
at roughly the same speed as before (speed may vary depending on your
computer). Though it will have required much more memory than a a normal Q16
IM (See
CompileTime Quality).
However such images, while more exactly representing the frequency components
of the FFT of the image, can contain negative and fractional values can only
be saved using special
HDRI capable File
Formats that can handle floatingpoint values.

Floating point compatible file formats include "MIFF ", "TIFF ", "PFM " and
the HDRI specific "EXR " file format. However you may need to
set "define quantum:format=floatingpoint " for it to work.

In later examples processing an FFT of an image, will need such accuracy to
produce good results. As such as we proceed with using Fast Fourier
Transforms, a
HDRI version ImageMagick will
become a requirement.
FFT as RealImaginary Components
So far we have only look at the 'Magnitude' and a 'Phase' representation of
Fourier Transformed images. But if you have compiled a
HDRI version of IM, you can also process images
using floating point 'Real' and 'Imaginary', components. This is done by using
the 'plus' versions of the options "
+fft
" and "
+ift
".
For example here I used a
HDRI version of IM to
also perform a 'round trip' FFT of an image, but this time generating
Real/Imaginary images.
# HDRI version of IM used
time convert lena.png +fft +ift lena_roundtrip_ri.png
echo n "RMSE = "
compare metric RMSE lena.png lena_roundtrip_ri.png null:
echo n "PAE = "
compare metric PAE lena.png lena_roundtrip_ri.png null:
 

You must use a HDRI version when you use the plus forms to generate
Real/Imaginary FFT images. If you don't, about 1/2 the values will be zero,
resulting in an image that looks 'dirty'. For example...
# nonHDRI Q16 version of IM used  THIS IS BAD
convert lena.png +fft +ift lena_roundtrip_ri_bad.png
 

The other thing to remember is that whichever form of FFT images you generate
will also effect ALL image processing operations you want to apply to the FFT
images. They are very different images, and as such they must be processed in
very different ways, with different mathematical operations.
Also as before you must have both Real and Imaginary component images to
restore the final image. For example, here is what happens if we substitute a
'black' image for one of the components.
# HDRI version of IM used
convert lena.png +fft delete 1 \
size 128x128 xc:black +ift lena_real_only.png
convert lena.png +fft delete 0 \
size 128x128 xc:black +ift lena_imaginary_only.png

Real Only

Imaginary Only

You can see from this that both Real/Imaginary FFT images contain vital
information about the original image fairly equally. The biggest difference
between the two is that the special DC or 'average color' has no imaginary
component and as such is only present in the magnitude image.
The diagonal mirror (actually a 180 rotation) effect you see in both images is
caused by the loss of the 'sign' information contained in the other component.
Without the other component, the wave could be thought to be 180 degrees out
of phase, and generating this weird look. This information loss is equal
between the two types of image.
Properties Of The Fourier Transform
FFT of a Constant Image
Lets demonstrate some of these properties.
First lets simply take a constant color image and get its magnitude.
convert size 128x128 xc:gold constant.png
convert constant.png fft +delete constant_magnitude.png

Note that the magnitude image in this case really is pureblack, except for
a single colored pixel in the very center of the image, at the pixel location
width/2, height/2. This pixel is the zero frequency or DC ('
Direct
Current') value of the image, and is the one pixel that does not represent
a sine wave. In other words this value is just the FFT constant component!
To see this single pixel more clearly lets also magnify that area of the
image...
convert constant_magnitude.png gravity center extent 5x5 \
scale 2000% constant_dc_zoom.gif
 

Note that the color of the DC point is the same as the original image.
Actually it is a good idea to remember that what you are seeing is three
values. That is the image generated is actually three separate Fast Fourier
transforms. A FFT for each of the three red, green and blue image channels.
The FFT itself has no real knowledge about colors, only the color values or
'
graylevels'.
In fact a FFT transform could be applied to just about any colorspace, as
really... It does not care! To a fourier transform a image is just an array
of values, and that is all.

While the 'phase' of the DC value is not important, it should always be
a 'zero' angle (a phase color value of 50% gray). If it is not set to 50%
gray, the DC value will have a 'unreal' component, and its value modulated
by the angle given.

Effects of the DC Color
In a more typical nonconstant image, the DC value is the average color of the
image. The color you should generally get if you had completely blurred,
averaged, or resized the image down to a single pixel or color.
For example lets extract the DC pixel from the FFT of the "Lena" image.
convert lena.png fft +delete lena_magnitude.png
convert lena_magnitude.png gravity center extent 1x1 \
scale 60x60 lena_dc_zoom.gif

As you can see the average color for the image is a sort of 'dark pink' color.
Another way of thinking about this special pixel is that it represents the
center 'bias' level, around which all the other sine waves modify the image
colors.
For example lets replace that 'dark pink' DC pixel with some other color
such as the more orange color 'tomato'...
convert lena.png fft \
\( clone 0 draw "fill tomato color 64,64 point" \) \
swap 0 +delete ift lena_dc_replace.png
 

What is really happening is that by changing the DC value in the FFT images
you are changing the whole image in that same way. Actually any change in the
DC value (the difference) will be added (or subtracted) from each and every
pixel in the resulting image.
This is just as if we really were really adding some constant to each pixel in
the original image. As such the final pixel colors in the reconstructed image
could also be clipped by the maximum (white) or minimum (black) limits. As
such this is not a recommended method of color tinting an image. This is
simpler to apply than modifying every pixel in the whole image, though the FFT
round trip will make it overall a much slower color tinting technique.
Spectrum Of A Sine Wave Image
Next, lets take a look at the spectrum from a single sine (or cosine) wave
image with 4 cycles across the image
convert size 128x129 gradient: chop 0x1 rotate 90 evaluate sine 4 \
sine4.png
convert sine4.png fft +delete \
autolevel evaluate log 100 sine4_spectrum.png


The unusual creation of the gradient image in the above is necessary to
ensure that the resulting sine wave image tiles perfectly across the image.
A normal "gradient: " image
does not perfectly tile, so neither does a sine wave generated from it.
A FFT transform of such a imperfect tile, will result in an array of
undesired harmonics, rather than single 'dots' in the Fourier Transform
Spectrum.
See Generating the Perfect
Gradient for more details about this problem.

In the spectrum image (enhanced magnitude image) above, we can see that it has
3 dots. The center dot is as before the average DC value. The other two dots
represent the perfect sine wave that the Fourier Operator found in the image.
As the frequency across the width of the image is exactly 4 cycles, and as
a result two frequency pixels are exactly 4 pixels away from the center DC
value.
But why two pixels?
Well that is because a sine single wave can be described in two completely
different ways, (one with a negative direction and phase). Both descriptions
are mathematically correct, and a fourier transform does not distinguish
between them.
If we repeat this with a sine wave with 16 cycles, then again we see that it
has 3 dots, but the dots are further apart. In this case the side dots are
spaced 16 pixels to the left and right sides of the center dot.
convert size 128x129 gradient: chop 0x1 rotate 90 evaluate sine 16 \
write sine16.png fft delete 1 \
autolevel evaluate log 100 sine16_spectrum.png

From this you can see that perfect sine waves will be represented simply by
two dots in the appropriate position. The distance this position is from the
center DC value determine the frequency of the sine wave. The smaller the
wave length the higher the frequency, so the further the dots will be from the
DC value.
In fact by dividing size of the image by the frequency (distance of the dots
from the center), will give you the wavelength (distance between peaks) of the
wave. In the above case: 128 pixels divided by 16 cycles, gives you
a wavelength of 8 pixels between each 'band'.
This is one of the most important distinguishing features of the FFT
transformation. A pattern of small features on the original image require
small wave lengths, and thus large frequencies. That results in large scale
effects in the frequency domain. Similarly large features, use smaller
frequencies, so generate small scale patterns, especially close in to toward
the center.
In a Fourier transform...
Small becomes large and large becomes small.
This is one of the most vital aspects to remember when dealing with Fourier
Transforms, as it is the key to removing noise (small features) from an image,
while preserving the overall larger aspects of the image.
Lets take a closer look at these three 'frequencies' by plotting their original
magnitudes (not the logarithmic spectrum).
convert sine16.png fft delete 1 miff: \
im_profile  sine16_magnitude_pf.png
 

Notice that the DC value (average or bias of image) has a value of 1/2 which
is to be expected (the average value of the image is a perfect 50% gray), but
that the actual magnitude of the two 16 cycle sinewaves the fourier transform
found is only 1/4 of the maximum value.
The magnitude of the original sinesave is really 1/2 but the fourier
transform divided that magnitude into two, sharing the results across both
plotted frequency waves, so each of the two components only has a magnitude of
1/4. That is a normal part of fourier transforms.
This duality of positive and negative frequencies in FFT images explains why
all FFT image spectrum's (such as the Lena spectrum repeated left) is always
symmetrical about the center. For every dot on one side of the image, you will
always get a similar 'dot' mirrored across the center of the image.
The same thing happens with the 'phase' component of FFT image pair, but with
a 180 degree shift (a negative phase) in value as well.
That means half each image is really a duplicate of the other half, but you
need BOTH images to recreate the original image. In other words the two
images still contain exactly the same amount of information, half in one
image, and half in the other. Together they produce a whole.

During generation the FFT algorithm only generates the left half the images.
The other half is generated by rotations and duplication of the generated
data.
When converting Frequency Domain images back to a Spatial Domain Image, the
algorithm again only looks at the left half of the image. The right half is
completely ignored, as it is only a duplicate.
As such when (in later examples) you 'notch filter' a FFT magnitude image,
you only really need to filter the left hand side of the magnitude image.
You can save yourself some work by also ignoring the right half. However
for clarity I will 'notch' both halves.

Generating FFT Images Directly
Now we can use the above information to actually generate a image of a sine
wave. All you need to do is create a black and 50% gray image pair, and
add 'dots' with the appropriate magnitude, and phase.
For example...
convert size 128x128 xc:black \
draw 'fill gray(50%) color 64,64 point' \
draw 'fill gray(50%) color 50,68 point' \
draw 'fill gray(25%) color 78,60 point' \
generated_magnitude.png
convert generated_magnitude.png \
autolevel evaluate log 3 generated_spectrum.png
convert size 128x128 xc:gray50 generated_phase.png
convert generated_magnitude.png generated_phase.png \
ift generated_wave.png

And presto a perfect angled (and tilable) sine wave.
Of course you can only generate perfect sine waves at particular frequencies,
and are only tilable in square images (unless resized later).
Unfortunately all the frequencies will also be a power of two in any
horizontal or vertical direction, and that is the main limitation of this
technique.

Actually only the first (left most) 'gray25' dot was needed to generate the
sine wave as the IFT transform completely ignores the right half of the image
as this should simply be a mirror of the left half.


The phase of the DC value must have a 'zero angle' (50% gray color). If you
don't ensure that is the case the DC color value will be modulated by its
nonzero phase, producing a darker, possibly 'clipped' image.


The other pixels in the phase can be any grey level you like, and will
effectively 'roll' the sine wave across the image. Again only the phase of
the left most dot actually matters. The right hand side is completely
ignored. Just ensure the center DC phase pixel remains 50% grey.

FUTURE: Perlin Noise Generator using FFT
Spectrum of a Vertical Line
Show the FFT spectrum of a thin and thick line
Demonstrate how small features become 'big' and big features become 'small'
in the FFT of the image. Link that back to the sine wave which could be
regarded as a 'line' with a single harmonic.
Rotate the line
Spectrum of a Rectangle Pattern Image
Next, lets look at the spectrum of white rectangle of width 8 and height 16
inside a black background.
convert size 8x16 xc:white gravity center \
gravity center background black extent 128x128 rectangle.png
convert rectangle.png fft +delete \
autolevel evaluate log 100 rect_spectrum.png

As you can see the resulting image has a very particular pattern, with a lot
of harmonic frequencies.
You can also see that the rectangle appeared to be rotated 90 degrees. That is
incorrect, what you are seeing is that same rule we mentioned before.. big
features become small and small features become big. As such the smaller
dimension of the rectangle became larger and the larger dimension became
smaller.
Now, lets rotate the rectangle by 45 degrees. We find that the spectrum is
also rotated in the same direction by 45 degrees.
convert rectangle.png rotate 45 gravity center extent 128x128 \
write rect_rot45.png fft delete 1 \
autolevel evaluate log 100 rect_rot45_spectrum.png

As you can see the same rotation in the frequency domain. That is the effect
of some rotated object will also be rotated in its Fourier Transform.
However if we now move the rectangle...
convert rectangle.png rotate 45 geometry +30+20 extent 128x128 \
write rect_rot45off.png fft delete 1 \
autolevel evaluate log 100 rect_rot45off_spectrum.png

The frequency pattern did not move. That is because all the positioning
information is contained in the phase image. The frequency pattern (magnitude
or its spectrum does not change because it moved).
This position separation, is one of the key features of the Fourier Transform
that makes it so very important. It will allow you to search for specific
image pattern within a larger image, regardless of the location of the object
that produced that fourier spectrum pattern.
Spectrum Of A Flat Circular Pattern Image
Next, lets look at the spectrum from an image with a white, flat circular
pattern, in one case with diameters of 12 (radius 6) and in another case
with diameter of 24 (radius 12).
convert size 128x128 xc:black fill white \
draw "circle 64,64 64,70" write circle6.png fft delete 1 \
autolevel evaluate log 100 circle6_spectrum.png
convert size 128x128 xc:black fill white \
draw "circle 64,64 64,76" write circle12.png fft delete 1 \
autolevel evaluate log 100 circle12_spectrum.png

Note that the first image is very close to what we generated for the jinc
example further above. It is however a little broken up. These artifacts
occurs due to the small size of the circle. Since it is represented digitally,
its perimeter is not perfectly circular. Again we see that the small details
become large in the transformed frequency space.
The transform of the larger circle is better as its perimeter is a closer
approximation of a true circle. We therefore conclude that indeed the
transform of the flat circular shape is a jinc function and that the image
containing the smaller diameter circle produces transform features that are
more spread out and wider.
According to the mathematical properties of a Fourier Transform, the distance
from the center to the middle of the first dark ring in the spectrum will be
1.22*N/d. When the diameter of the circle is d=12, we get a distance of
1.22*128/12=13. Likewise when the diameter of the circle is d=24, we get
a distance of 1.22*128/24=6.5.
Spectrum Of A Gaussian Pattern Image
Next, lets look at the spectrum from two images, each with a white Gaussian
circular pattern having sigmas of 8 and 16, respectively
convert size 128x128 xc:black fill white \
draw "point 64,64" gaussianblur 0x8 autolevel \
write gaus8.png fft delete 1 \
autolevel evaluate log 1000 gaus8_spectrum.png
im_profile s gaus8.png gaus8_pf.gif
im_profile s gaus8_spectrum.png gaus8_spectrum_pf.gif

convert size 128x128 xc:black fill white \
draw "point 64,64" gaussianblur 0x16 autolevel \
write gaus16.png fft delete 1 \
autolevel evaluate log 1000 gaus16_spectrum.png
im_profile s gaus16.png gaus16_pf.gif
im_profile s gaus16_spectrum.png gaus16_spectrum_pf.gif

Other than the noise produced by the rectangular array of the pattern, the
result was that a Gaussian pattern produced a almost identical Gaussian
frequency pattern. More importantly that pattern was quite clean in
appearance.
Of course there is a size difference, again following that same rule, big
becomes small and small become big.
From the mathematical properties, the sigma in the spectrum will be just
N/(2*sigma), where sigma is from the original image. So for an image of size
N=128 and sigma=8, the sigma in the spectrum will be 128/16=8. Similarly if the
image's sigma is 16, then the sigma in the spectrum will be 128/32=4.
This is the mathematical relationship of the "big becomes small and
visaversa" rule, and it can be useful to know.
Spectrum Of A Grid Pattern Image
Next, lets transform an image containing just a set of grid lines spaced 16x8
pixels apart.
convert size 16x8 xc:white fill black \
draw "line 0,0 15,0" draw "line 0,0 0,7" \
write mpr:tile +delete \
size 128x128 tile:mpr:tile \
write grid16x8.png fft delete 1 \
autolevel evaluate log 100000 grid16x8_spectrum.png

The resulting spectrum is just an array of dots where the grid
lines that are more closely spaced produce dots further apart and vice versa.
According to the properties above, since the grid lines are spaced 16x8 pixels
apart, then the dots should be spaced N/a=128/16=8 and M/b=128/8=16, which is
just what is measured in this image.
This pattern is of particular importance as it will let you know the
relationship for the Fourier Transform to a regular tiling pattern in an
image. Such a tiling patterns produces very strong noncentral grid patterns
in its Fourier Transform.
The key point here is that the shape information is in the center, but tiling
information is in a grid like array away from the center of its fourier
transform.
More Spectrum Information
Here are some links if you like to know more about spectrum images and there
properties.
Practical Applications
OK, now that we have covered the basics, what are the practical applications
of using the Fourier Transform?
Some of the things that can be done include: 1) increasing or decreasing the
contrast of an image, 2) blurring, 3) sharpening, 4) edge detection and
5) noise removal.
Changing The Contrast Of An Image  Coefficient Rooting
One can adjust the contrast in an image by performing the forward Fourier
transform, raising the magnitude image to a power and then using that with the
phase in the inverse Fourier transform. To increase, the contrast, one uses an
exponent slightly less than one and to decrease the contrast, one uses an
exponent slightly greater than one. So lets first increase the contrast on the
Lena image using an exponent of 0.9 and then decrease the contrast using an
exponent of 1.1.
convert lena.png fft \
\( clone 0 evaluate pow 0.9 \) delete 0 \
+swap ift lena_plus_contrast.png
convert lena.png fft \
\( clone 0 evaluate pow 1.1 \) delete 0 \
+swap ift lena_minus_contrast.png

However doing this to the original image would also have the same effect as
doing this to the original image. That is a global modification of the
magnitudes has the same effect as if you did a global modification of the
original image.
Blurring An Image  Low Pass Filtering
One of the most important properties of Fourier Transforms is that convolution
in the spatial domain is equivalent to simple multiplication in the frequency
domain. In the spatial domain, one uses small, squaresized, simple
convolution filters (kernels) to blur an image with the
convole option. This is called a low
pass filter. The simplest filter is just a an equallyweighted, square array.
That is all the values are ones, which are normalized by dividing by their sum
before applying the convolution. This is equivalent to a local or
neighborhood average. Another low pass filter is the Gaussianweighted,
circularly shaped filter provided by either
gaussianblur or
blur.
In the frequency domain, one type of low pass blurring filter is just
a constant intensity white circle surrounded by black. This would be similar
to a circularly shaped averaging convolution filter in the spatial domain.
However, since convolution in the spatial domain is equivalent to
multiplication in the frequency domain, all we need do is perform a forward
Fourier transform, then multiply the filter with the magnitude image and
finally perform the inverse Fourier transform. We note that a small sized
convolution filter will correspond to a large circle in the frequency domain.
Multiplication is carried out via
composite with a
compose
multiply setting.
So lets try doing this with two sizes of circular filters, one of diameter 40
(radius 20) and the other of diameter 28 (radius 14).
convert size 128x128 xc:black fill white \
draw "circle 64,64 44,64" circle_r20.png
convert lena.png fft \
\( clone 0 circle_r20.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_blur_r20_spec.png +delete \) \
swap 0 +delete ift lena_blur_r20.png
convert size 128x128 xc:black fill white \
draw "circle 64,64 50,64" circle_r14.png
convert lena.png fft \
\( clone 0 circle_r14.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_blur_r14_spec.png +delete \) \
swap 0 +delete ift lena_blur_r14.png

So we see that the image that used the smaller diameter filter produced more
blurring. We also note the 'ringing' or 'ripple' effect near edges in the
resulting images. This occurs because the Fourier Transform of a circle, as we
saw earlier, is a jinc function, which has decreasing oscillations as it
progresses outward from the center. Here however, the jinc function and the
oscillations are in the spatial domain rather than in the frequency domain, as
we had demonstrated earlier above.
So what can we do about this? The simplest thing is to taper the edges of the
circles using various
Windowing Functions. Alternately, one can use a filter
such as a Gaussian shape that is already by definition tapered.
So lets do the latter and use two Gaussian blurred circles to remove
most of the sever 'ringing' effects.
convert circle_r20.png blur 0x4 autolevel gaussian_r20.png
convert lena.png fft \
\( clone 0 gaussian_r20.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_gblur_r20_spec.png +delete \) \
swap 0 +delete ift lena_gblur_r20.png
convert circle_r14.png blur 0x4 autolevel gaussian_r14.png
convert lena.png fft \
\( clone 0 gaussian_r14.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_gblur_r14_spec.png +delete \) \
swap 0 +delete ift lena_gblur_r14.png

This of course is much better.
The ideal lowpass filter is not to blur circles at all, but actually use a
proper gaussian curve of
sigma rather than a
radius.
Of course in this example we ended up doing a blur, to do a blur! However the
blur pattern that is multiplied against the FFT magnitude image used is fixed,
and could in fact be retrieved from a pregenerated cache. Also the
multiplying image does not need to be the full size of the original image, you
can use a smaller image. As such the above can be a lot faster for large
images, and in the case of handling lots of images.
The more important point is for large strong blurs, the frequency domain image
is small, and only does a single multiply, rather then having to average lots
of pixels, for each and every pixel in the original image.
For small sized blurs you may be better with the more direct convolution blur.
Detecting Edges In An Image  High Pass Filtering
In the spatial domain, high pass filters that extract edges from an image are
often implemented as convolutions with positive and negative weights such that
they sum to zero.
Things are much simpler in the frequency domain. Here a high pass filter is
just the negated version of the low pass filter. That is where the low pass
filter is bright, the high pass filter is dark and vice versa. So in
ImageMagick, all we need do is to
negate the low pass filter image.
So lets apply high pass filters to the Lena image using a circle image.
And then again using a purely gaussian curve.
convert circle_r14.png negate circle_r14i.png
convert lena.png fft \
\( clone 0 circle_r14i.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_edge_r14_spec.png +delete \) \
delete 0 +swap ift normalize lena_edge_r14.png
convert size 128x128 xc: draw "point 64,64" blur 0x14 \
autolevel gaussian_s14i.png
convert lena.png fft \
\( clone 0 gaussian_s14i.png compose multiply composite \) \
\( +clone evaluate log 10000 write lena_edge_s14_spec.png +delete \) \
delete 0 +swap ift normalize lena_edge_s14.png

Carefully examining these two results, we see that the simple circle is not
quite as good as the gaussian, as it has 'ringing' artifacts and is not quite
as sharp.
Sharpening An Image  High Boost Filtering
The simplest way to sharpen an image is to high pass filter it (without
the normalization stretch) and then blend it with the original image.
convert lena.png fft \
\( size 128x128 xc: draw "point 64,64" blur 0x14 autolevel \
clone 0 compose multiply composite \) \
delete 0 +swap ift \
lena.png compose blend set option:compose:args 100x100 composite \
lena_sharp14.png

Here a high pass filter, is done in the frequency domain and the result
transformed back to the spatial domain where it is blended with the original
image, to enhance the edges of the image.
Noise Removal  Notch Filtering
Many noisy images contain some kind of patterned noise. This kind of noise is
easy to remove in the frequency domain as the patterns show up as either a
pattern of a few dots or lines. Recall a simple sine wave is a repeated
pattern and shows up as only 3 dots in the spectrum.
In order to remove this noise, one simply, but unfortunately, has to manually
mask (or notch) out the dots or lines in the magnitude image. We do this by
transforming to the frequency domain, create a grayscale version of the
spectrum, mask the dots or lines, threshold it, multiply the binary mask image
with the magnitude image and then transform back to the spatial domain.
Lets try this on the
clown image, which contains a diagonally striped
ditherlike pattern. First we transform the clown image to create its
magnitude and phase images.
convert clown.jpg fft \
\( +clone write clown_phase.png +delete \) +delete \
write clown_magnitude.png colorspace gray \
autolevel evaluate log 100000 clown_spectrum.png

We see that the spectrum contains four bright starlike dots, one in each
quadrant. These unusual points represent the pattern in the image we want to
get rid of.
The bright dot and lines in the middle of the image are of no concern as they
represent the DC (average image color), and effects of the edges from the
image, and should not be modified.
Note that when generating the spectrum image I forced the resulting image to
be a pure grayscale image. This is so I can now loaded the image into an
editor, and using any nongray color (such as red), I masked out the area of
those 4 star like patterns.
When finished editing I can extract the areas I colored, by extracting
a difference image against the unedited version. Like this...
convert clown_spectrum_edited.png clown_spectrum.png \
compose difference composite \
threshold 0 negate clown_spectrum_mask.png

Now we simply multiply the mask with the magnitude and use the result with the
original phase image to transform back to the spatial domain. We display the
original image next to it for comparison
convert clown_magnitude.png clown_spectrum_mask.png \
compose multiply composite \
clown_phase.png ift clown_filtered.png

A very good result. But we can do even better.
As you saw in the previous examples, simple 'circles' are not particularly
friendly to a FFT image, so lets blur the mask slightly...
convert clown_spectrum_mask.png \
blur 0x5 level 50x100% clown_mask_blurred.png
 

And filter the clown, this time regenerating the FFT images in memory.
convert clown.jpg fft \
\( clone 0 clown_mask_blurred.png compose multiply composite \) \
swap 0 +delete ift clown_filtered_2.png
 

A simply amazing result! And one that could possibly be improved further by
adjusting that mask to fit the 'star' shapes better.
We can even take the difference between the original and the result to create
an image of the areas where noise was removed.
convert clown.jpg clown_filtered_2.png compose difference \
composite normalize clown_noise.png
 

Lets try this on an another example. This time on a "Twigs" image found on the
RoboRealm website, which
contains an irregular pattern of horizontal and vertical stripes.
Again we extract a greyscale spectrum image, just as we did before.
convert twigs.jpg fft +delete colorspace gray \
autolevel evaluate log 100000 twigs_spectrum.png

In this case, as the noise in the image is horizontally and vertically
oriented, this shows up as thick horizontal and vertical bands along the
center lines but not in the actual center of the image.
Again we mask out the parts using a image editor, this time using a 'blue'
color (it really doesn't matter which color is used)...
convert twigs_spectrum_edited.png twigs_spectrum.png \
compose difference composite \
threshold 0 negate twigs_spectrum_mask.png

Now we again multiply the mask with the FFT magnitude image, and reconstruct
the image.
convert twigs.jpg fft \
\( clone 0 twigs_spectrum_mask.png compose multiply composite \) \
swap 0 +delete ift twigs_filtered.png

And we can take the difference between the original and the result to create
an image of the areas where noise was removed.
convert twigs.jpg twigs_filtered.png compose difference composite \
normalize twigs_noise.png

Adding a little blur to the mask, could again improve the results even more.
As a exercise, try removing the string from the image. As a hint remember how
the effect of a line in a real image is rotated 90 degrees in the FFT. If you
get this wrong, you'll probably remove the twig instead.
Advanced Applications
Some of the other more advanced applications of using the Fourier Transform
include: 1) deconvolution (deblurring) of motion blurred and defocused images
and 2) normalized cross correlation to find where a small image best matches
within a larger image.
Examples of FFT Multiplication and Division (deconvolution) moved to a
subdirectory as it is waiting a more formally defined
image processing operators.